Sound detecting mechanism

ABSTRACT

A sound detecting mechanism is provided which forms a diaphragm with a required thickness by thickness control and yet restrains distortion of the diaphragm to provide high sensitivity. 
     The sound detecting mechanism comprises a pair of electrodes forming a capacitor on a substrate A in which one of the electrodes is a back electrode C forming perforations Ca therein corresponding to acoustic holes and the other of the electrodes is a diaphragm B. The diaphragm B is mounted on the substrate A while the back electrode C is mounted in a position opposed to the diaphragm B across a void F to be supported by the substrate A, the back electrode C being formed by polycrystal silicon of 5 μm to 20 μm in thickness.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sound detecting mechanism comprisinga pair of electrodes forming a capacitor on a substrate, in which one ofthe electrodes is a back electrode forming perforations thereincorresponding to acoustic holes and the other of the electrodes is adiaphragm.

2. Description of Related Art

Conventionally, condenser microphones are frequently used in mobilephones, for example. A typical construction of condenser microphones isshown in FIG. 6. This condenser microphone comprises a metal capsule 100including a plurality of perforations “h” corresponding to acousticholes formed therein, a fixed electrode 300 and a diaphragm 500 providedinside the capsule to be opposed to each other with a spacer 400therebetween to maintain a predetermined gap, a substrate 600 fixed andfitted to a rear opening of the capsule 100, and an impedance convertingelement 700 made of J-FET or the like and mounted to the substrate 600.With this type of condenser microphone, a high voltage is applied to adielectric material formed on the fixed electrode 300 or the diaphragm500 to be heated to generate electric polarization and produce anelectret membrane allowing a residual electric charge to remain on asurface thereof (an electret membrane 510 is formed in a diaphragm body520 made of metal or conductive film which constitutes the diaphragm 500in FIG. 6), thereby to provide a construction that requires no biasvoltage. When the diaphragm 500 is vibrated by sound pressure signals ofa sound, a distance between the diaphragm 500 and the fixed electrode300 is changed to vary capacitance. The variation of capacitance isoutputted through the impedance converting element 700.

Another conventional sound detecting mechanism has the followingconstruction. This sound detecting mechanism comprises a substrate (110)constituting a diaphragm and a substrate (108) constituting a back faceplate (103) (corresponding to the back electrode of the presentinvention), both substrates being superimposed through an adhesive layer(109) and then adhered to each other through heat treatment. Then, thesubstrate (108) acting as the back face plate is ground to obtain adesired thickness. After an etching mask (112) is formed on each of thesubstrates (108) and (109), the substrates are treated with an alkalietching liquid thereby to obtain the diaphragm (101) and the back faceplate (103). Next, the back face plate (103) is reticulated(corresponding to the perforations of the present invention). Aninsulating layer (111) is etched with hydrofluoric acid, with the backface plate (103) acting as an etching mask, thereby to form a void layer(104) (see Patent Document 1, for example: the reference numbers arequoted from the cited document.)

Patent Document 1: Japanese Patent Publication No. 2002-27595 (paragraph[0030] through [0035], FIG. 1 and FIG. 3).

In order to increase output (improve sensitivity) of the conventionalmicrophone shown in FIG. 6, it is required to increase the capacitancebetween the fixed electrode 300 and the diaphragm 500. In order toincrease the capacitance, an area of superimposition of the fixedelectrode 300 and the diaphragm 500 should be increased. Alternatively,it will be effective to reduce the gap between the fixed electrode 300and the diaphragm 500. However, an increase in the area ofsuperimposition of the fixed electrode 300 and the diaphragm 500 wouldlead to an enlargement of the microphone per se. On the other hand, inthe construction having the spacer 400 noted above, there is alimitation in reducing the distance between the fixed electrode 300 andthe diaphragm 500.

Also, the electret condenser microphones often utilize a high polymericorganic substance such as FEP (Fluoro Ethylene Propylene) or the like inorder to produce a permanent electric polarization. The microphone usingsuch a high polymeric organic substance has poor heat resistance, andthus is hardly capable of enduring the heat in time of re-flow treatmentwhen mounted on a printed board, for example. The microphone, therefore,cannot be given re-flow treatment when mounted on the printed board orthe like.

In view of the above, as described in Patent Document 1, it isconceivable to employ a construction including a back electrode and adiaphragm formed on a silicon substrate by micro fabrication technique.A sound detecting mechanism having such a construction is compact andyet is capable of enhancing sensitivity by reducing the distance betweenthe back electrode and the diaphragm. Further, the mechanism can undergore-flow treatment while requiring a bias supply. However, according tothe technique set forth in Patent Document 1, the diaphragm is formed byetching a monocrystal silicon substrate with an alkali etching liquid,which makes it difficult to control the thickness of the diaphragm. As aresult, it is difficult to obtain a required thickness for thediaphragm.

In considering control of the thickness of the diaphragm here, it iseffective to utilize a single crystal silicon on insulator (SOI) waferto improve the controllability of the thickness of the diaphragm in theprocess of forming the diaphragm by etching the silicon substrate withthe alkali etching liquid. More particularly, according to this method,a built-in oxide film of the SOI wafer can be utilized as a stop layerfor etching with the alkali etching liquid, thereby to control thethickness of the diaphragm by selecting the thickness of an active layerof the SOI wafer.

As a different method to the above, it is conceivable to utilize,instead of using the SOI wafer, an SOI structure wafer in which siliconoxide film or silicon nitride film is formed on the monocrystal siliconsubstrate as an etching stop layer to function as a stop layer in timeof etching with the alkali etching liquid, and polycrystal silicon isformed on the etching stop layer. With this SOI structure wafer, itbecomes possible to stop etching with the etching stop layer when thesilicon substrate is etched with the alkali etching liquid, thereby toenhance the controllability of the thickness of the diaphragm.

However, in the method utilizing the SOI wafer or the method utilizingthe SOI structure wafer, the sound detecting mechanism has aconstruction with a plurality of materials (films or layers) laminatedon monocrystal silicon acting as a base. Thus, while a relatively thindiaphragm can be formed with high accuracy by stopping etching with theetching stop layer to form the diaphragm, the diaphragm is distorted byan inner stress caused by a difference between coefficients of thermalexpansion of the plurality of materials laminated on the monocrystalsilicon, which might lead to a disadvantage of deteriorating thevibration characteristic to hamper vibrations faithful to sound pressuresignals when the diaphragm contacts the back electrode or even when thediaphragm does not contact the back electrode.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a rationalconstruction for a sound detecting mechanism having a diaphragm with arequired thickness by thickness control and yet restraining distortionof the diaphragm to realize high sensitivity.

The first characteristic feature of a sound detecting mechanismaccording to the present invention lies in comprising a pair ofelectrodes forming a capacitor on a substrate in which one of theelectrodes is a back electrode forming perforations thereincorresponding to acoustic holes and the other of the electrodes is adiaphragm, wherein the diaphragm is mounted on the substrate while theback electrode is mounted in a position opposed to the diaphragm acrossa void to be supported by the substrate, the back electrode being formedby polycrystal silicon of 5 μm to 20 μm in thickness.

[Function and Effect]

According to the above-noted construction, as in the construction wherea relatively thin diaphragm is formed by etching the substrate having anetching stop layer formed thereon, for example, even when a stresscaused by a difference of coefficients of thermal expansion between aplurality of materials forming the etching stop layer, the diaphragm andthe like, acts on the diaphragm, the thickness of the back electrodeformed in the position opposed to the diaphragm is selected to be arelatively large value, between 5 μm to 20 μm, thereby to enhance themechanical strength of the diaphragm and restrain distortion of thediaphragm caused by the internal stress. Therefore, this will not causeany disadvantage that the diaphragm contacts the back electrode or thelike. In the microphone shown in FIG. 1 as a specific construction (seeBest Mode For Carrying Out the Invention for details such as thethickness of the film), as illustrated in FIG. 4, the thickness of theback electrode C (film thickness of the back electrode) is selected tobe within a range of 5 μm to 10 μm to restrain the bending amount of thediaphragm B to 3 μm or less while the thickness of the back electrode Cis selected to be within a range of 15 μm to 20 μm to restrain thebending amount of the diaphragm B to 1 μm or less. Further, theabove-noted characteristic feature provides the construction thatdispenses with an electret layer and is capable of enduring the heat intime of re-flow treatment when mounted on a printed board. As a result,even if the diaphragm has a reduced thickness by employing a simpleconstruction of selecting the thickness of the back electrode, thephenomenon that distorts the diaphragm by the internal stress can beavoided and the sound detecting mechanism having high sensitivity andcapable of enduring the heat in time of re-flow treatment can beprovided. In particular, the construction where the thickness of theback electrode is selected to have a relatively large value as in thepresent invention may also provide an effect that a frequencycharacteristic such as resonance frequency can be controlled byappropriately selecting the thickness of the back electrode.

The second characteristic feature of the sound detecting mechanismaccording to the present invention lies in that the substrate comprisesa support substrate having a monocrystal silicon substrate acting as thebase thereof, and that a silicon substrate of (100) orientation is usedas the monocrystal silicon substrate.

[Function and Effect]

According to the above-noted construction, it is possible to promoteetching selectively in a direction of the orientation peculiar to themonocrystal silicon substrate of (100) orientation, which allows forfine etching faithful to an etching pattern. As a result, a requiredshaping process is realized.

The third characteristic feature of the sound detecting mechanismaccording to the present invention lies in that impurity diffusiontreatment is executed on the diaphragm.

[Function and Effect]

According to the above-noted construction, impurity diffusion treatmentis executed on the diaphragm thereby to control the stress of thediaphragm and further control the tension of the diaphragm bycontrolling the stress. As a result, a distortion of the diaphragm canbe efficiently eliminated. In particular, in this construction, aneffect is produced that a distortion of the diaphragm can be furtherefficiently eliminated by combination of the thickness of the diaphragmand the thickness of the back electrode.

The fourth characteristic feature of the sound detecting mechanismaccording to the present invention lies in that the substrate comprisesa support substrate having a monocrystal silicon substrate acting as thebase thereof, and that the support substrate consists of an SOI wafer.

[Function and Effect]

According to the above-noted construction, a built-in oxide film formedon the SOI wafer can be utilized as a stop layer for etching with analkali etching solution by treatment executed on the SOI wafer. Also, itis possible to use a film already formed on the SOI wafer as thediaphragm or use a newly formed film as the diaphragm. As a result, theSOI wafer having the necessary film already formed thereon is used toreadily provide the sound detecting mechanism.

The fifth characteristic feature of the sound detecting mechanismaccording to the present invention lies in that the SOI wafer has anactive layer used as the diaphragm.

[Function and Effect]

According to the above-noted construction, the active layer alreadyformed on the SOI wafer is used as the diaphragm, which dispenses withtreatment for forming the diaphragm. As a result, the sound detectingmechanism can be readily provided without forming a new filmrepresenting the diaphragm.

The sixth characteristic feature of the sound detecting mechanismaccording to the present invention lies in that the diaphragm is formedof monocrystal silicon of 0.5 μm to 5 μm in thickness.

[Function and Effect]

According to the above-noted construction, a relatively thin diaphragmhaving the thickness of 0.5 μm to 5 μm is formed by using themonocrystal silicon based on the technique established for manufacturingan integrated circuit, thereby to allow the diaphragm to be vibrated ingood response to sound pressure signals. As a result, the sounddetecting mechanism having high sensitivity is provided.

The seventh characteristic feature of the sound detecting mechanismaccording to the present invention lies in that the support substrateconsists of an SOI structure wafer including a silicon oxide film or asilicon nitride film formed on a monocrystal silicon substrate and apolycrystal silicon film formed on the silicon oxide film or the siliconnitride film.

[Function and Effect]

According to the above-noted construction, in the case where thepolycrystal silicon film is formed on a top surface of the silicon oxidefilm or the silicon nitride film formed on the monocrystal siliconsubstrate, the silicon oxide film or the silicon nitride film can beused as the etching stop layer when the polycrystal silicon film or anyfilm formed on an external surface thereof is formed as the diaphragm byetching the monocrystal silicon. As a result, the diaphragm may readilyhave a reduced thickness by selecting the film thickness, thereby toprovide the sound detecting mechanism of high sensitivity. Inparticular, in the construction where the diaphragm is formed by thepolycrystal silicon formed on an outer layer than the oxide silicon withthe monocrystal silicon substrate being the base while the backelectrode is formed by the polycrystal silicon having a sacrificiallayer consisting of oxide silicon disposed in an outer side of thediaphragm, stress acts in a compressing direction caused by thecoefficients of thermal expansion of the films other than thepolycrystal silicon film forming the diaphragm with the reference to thecoefficient of thermal expansion of the back electrode (polycrystalsilicon). Since the silicon nitride film has a property of exerting thestress in a stretching direction, an effect can be provided to alleviatethe stress acting on the diaphragm by balancing the stress in thecompressing direction and the stress in the stretching direction byforming the silicon nitride film.

The eighth characteristic feature of the sound detecting mechanismaccording to the present invention lies in that the polycrystal siliconfilm formed on the SOI structure wafer is used as the diaphragm.

[Function and Effect]

According to the above-noted construction, since the polycrystal siliconfilm is used as the diaphragm, the diaphragm can be formed using thefilm formed on the SOI structure wafer without forming a special film.As a result, the processing steps in manufacture are reduced thereby toreadily provide the sound detecting mechanism.

The ninth characteristic feature of the sound detecting mechanismaccording to the present invention lies in that the diaphragm is formedby the polycrystal silicon of 0.5 μm to 5 μm in thickness.

[Function and Effect]

According to the above-noted construction, it is possible to form thediaphragm having a relatively small thickness using the polycrystalsilicon based on the technique established for manufacturing integratedcircuits. As a result, the sound detecting mechanism having highsensitivity is realized.

BRIEF DESCRIPTION OF THE DRAWINGS

-   -   FIG. 1 shows a sectional view of a condenser microphone;    -   FIG. 2 shows a view consecutively showing steps for        manufacturing the condenser microphone;    -   FIG. 3 shows view consecutively showing steps for manufacturing        the condenser microphone;    -   FIG. 4 shows a graphic presentation showing a relationship        between thickness of a back electrode and amount of bending of a        diaphragm;    -   FIG. 5 shows a graphic presentation showing a relationship        between thickness of the back electrode and damage rate of a        structure; and    -   FIG. 6 shows a sectional view of a conventional condenser        microphone.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will be described hereinafterwith reference to the drawings.

FIG. 1 is a sectional view of a silicon condenser microphone (simplyreferred to as a microphone hereinafter) exemplifying a sound detectingmechanism of the present invention. The microphone comprises a supportsubstrate A having a base of monocrystal silicon, a diaphragm B and aback electrode C formed on the support substrate A from polycrystalsilicon film made, and a sacrificial layer made of silicon oxide film(SiO₂) and arranged between the diaphragm B and the back electrode C toact as spacer D. This microphone allows the diaphragm B and the backelectrode C to function as a capacitor, which is used to electricallytake out variations of capacitance of the capacitor when the diaphragm Bis vibrated by sound pressure signals.

The support substrate A in this microphone has a size of a square withone side 5.5 mm in length and around 600 μm in thickness. The diaphragmB has a size of a square with one side 2.0 mm in length and around 2 μmin thickness. The back electrode C has a plurality of perforations Caformed therein corresponding to acoustic holes, each having a squarewith one side around 10 μm in length. In FIG. 1, the thickness of partof the films or layers is shown in an exaggerated way.

The microphone includes an SOI structure wafer having a silicon oxidefilm 302 and a polycrystal silicon film 303 formed on a top surface ofthe monocrystal silicon substrate 301. On a top surface of the SOIstructure wafer are formed a sacrificial layer 305 and a polycrystalsilicon film 306. The top polycrystal silicon film 306 undergoes etchingto form the back electrode C and the plurality of perforations Ca.Further, etching is executed on a portion extending from the backsurface of the monocrystal silicon substrate 301 through the polycrystalsilicon film 303 to form an acoustic opening E. The diaphragm B isformed by the polycrystal silicon film 303 exposed to the portion of theacoustic opening E, and further the sacrificial layer 305 undergoesetching to define a void area F between the diaphragm B and the backelectrode C. The spacer D is formed by the sacrificial layer 305remaining at outer peripheral portions of the diaphragm B after theetching. Steps for manufacturing the microphone will be described basedon FIGS. 2( a) through 2(e), and FIGS. 3( f) through 3(j).

Step (a): The silicon oxide films 302 (SiO₂) of 0.8 μm in thickness areformed by thermal oxidation and the polycrystal silicon films 303 of 2μm in thickness are formed by LP-CVD (Low Pressure Chemical VaporDeposition) technique on opposite surfaces of the monocrystal siliconsubstrate 301 of (100) orientation having a thickness of 600 μm, therebyto form the support substrate A representing the SOI structure wafer.

According to the present invention, the SOI structure wafer is notlimited to the construction shown in step (a) above. Instead, an SOIstructure wafer may be used in which a silicon nitride film (Si₃N₄) isformed on the monocrystal silicon substrate 301 and the polycrystalsilicon film 303 is formed on a top surface of the silicon nitride film.Also, the thickness of the polycrystal silicon film 303 is not limitedto 2 μm, but may be within the range of 0.5 μm to 5 μm.

Step (b): The silicon oxide film (SiO₂) functioning as the sacrificiallayer 305 of 5 μm in thickness is formed on a top surface of the supportsubstrate A (upper side in the drawings) formed in step (a) by P-CVD(Plasma Chemical Vapor Deposition) technique.

Step (c): A polycrystal silicon film 306 of 5 μm to 20 μm in thicknessis formed on a surface of the sacrificial layer 305 formed in step (b)by P-CVD technique. The polycrystal silicon film 306 constitutes theback electrode C and is formed on each of the opposite surfaces of thesubstrate.

Step (d): Photoresist is applied to the surface of the polycrystalsilicon film 306 formed in step (c), and a resist pattern 307 is formedby removing unwanted portions by photolithographic technique.

Step (e): Etching is executed by RIE (Reactive Ion Etching) technique,using the resist pattern 307 formed in step (d) as a mask, to form apattern of the back electrode C from the upper polycrystal silicon film306. The plurality of perforations Ca are simultaneously formed when thepattern of the back electrode C is formed in this way. Also, the backside (lower side in the drawings) polycrystal silicon film 306 and theinner polycrystal silicon film 303 are removed by executing etching inthis way.

Steps (f), (g): Next, a silicon nitride film 309 is formed on the backside (lower side in the drawings). Photoresist is applied to the surfaceof this film to form a resist pattern by removing unwanted portions byphotolithographic technique. Then, etching is executed by RIE (ReactiveIon Etching) technique, using the resist pattern as a mask, to removethe silicon nitride film 309 and the silicon oxide film 302 which is aninner layer of the film 309. This produces an opening pattern 310 forsilicon etching which realizes etching with an alkali etching solutionexecuted in a step (i) described later.

Steps (h), (i): Next, a silicon nitride film 311 (Si₃N₄) is formed onthe top side which acts as protective film. Subsequently, anisotropicetching is executed from the back side using an aqueous solution of TMAH(tetramethylammonium-hydroxide) as an etching solution to remove thesilicon substrate 301, thereby to form the acoustic opening E. In thisetching process, since the rate of etching the silicon oxide film 302(built-in oxide film) is sufficiently lower than the rate of etching thesilicon substrate 301, the silicon oxide film 302 functions as the stoplayer for silicon etching.

Step (j): Next, the nitride film 311 (Si3N4) formed as the protectivelayer, the sacrificial layer 305, the silicon oxide film 302 exposed tothe acoustic opening E, the silicon nitride film 309 and the siliconoxide film 302 remaining on the back side of the silicon substrate areremoved by etching with HF (hydrogen fluoride). This results in thediaphragm B formed by the polycrystal silicon film 303, the void area Fformed between the diaphragm B and the back electrode C, and the spacerD formed by the remaining sacrificial layer 305. Subsequently, Au (gold)is vapor-deposited to desired positions using a stencil mask to form atake-out electrode 315, thereby to complete the microphone.

With respect to the microphone manufactured according to the aboveprocess in which the thickness of the polycrystal silicon film 306acting as the back electrode C is varied, FIG. 4 shows results ofmeasuring amounts of bending of the diaphragm B by a laser displacementgauge. As shown, it can be understood that the bending amount of thediaphragm B is controlled to have tendencies to decrease with anincrease in the thickness of the back electrode C. In particular, it canbe understood that the bending amount of the diaphragm B is restrainedto 3 μm or less by selecting the thickness of the back electrode C to bewithin the range of 5 μm to 10 μm, while the bending amount of thediaphragm B is restrained to 1 μm or less by selecting the thickness ofthe back electrode C to be within the range of 15 μm to 20 μm.

As described above, the sound detecting mechanism according to thepresent invention employs the construction including the diaphragm B andthe back electrode C formed on the support substrate A by utilizingmicro fabrication technique. As a result, the entire sound detectingmechanism may be made quite compact and readily incorporated to smalldevices such as mobile phones. Moreover, it is capable of enduringre-flow treatment at high temperature when it is mounted on a printedboard, which makes it easy to assemble the apparatus.

Particularly, in the construction where the diaphragm B is formed byetching the support substrate A as in the present invention, themicrophone with high sensitivity can be obtained by reducing thethickness of the diaphragm B. Since coefficients of thermal expansion ofthe materials constituting the plural films or layers formed on thesupport substrate A are different, the stress caused by the differencebetween the coefficients of thermal expansion is exerted on thediaphragm B in a compressing direction when the microphone is completed.As in the present invention, the polycrystal silicon film 306 is usedfor the back electrode C arranged in a position corresponding to thediaphragm B, and the back electrode C has an increased thickness(specifically 5 μm to 20 μm), thereby to enhance the mechanical strengthof the diaphragm B as well as restrain distortion of the diaphragm Beven when a force is exerted in a direction to distort the diaphragm Bresulting from the internal stress. Also, the phenomenon to distort thediaphragm B by the internal stress can be avoided even if the diaphragmhas a reduced thickness, thereby providing the microphone of highsensitively (one example of the sound detecting mechanism).

[Modified Embodiments]

Apart from the above-described embodiment, the present invention may beimplemented as follows (common reference numbers and signs being usedfor the components in the following modified embodiments that have thesame functions as in the foregoing embodiment).

(1) In the forgoing embodiment, the SOI structure wafer is used as thesupport substrate A, in which the silicon oxide film 302 is formed onthe monocrystal silicon substrate 301, and then the polycrystal siliconfilm 303 is formed on the silicon oxide film 302. Instead, an SOI wafermay be used as the support substrate A including the active layer formedon an external surface of a built-in oxide film. Further, the activelayer may represent the diaphragm B in the SOI wafer having he activelayer, while the monocrystal silicon film may represent the diaphragm Bin the SOI wafer having the monocrystal silicon film formed thereon. Inparticular, when the diaphragm B is formed by the monocrystal siliconfilm, it is desirable that the film thickness is selected to be withinthe range of 0.5 μm to 5 μm to obtain good sensitively.

(2) With respect to the silicon condenser microphones utilizing the SOIwafer as the support substrate A in which the thickness of the backelectrode C is varied, FIG. 5 shows results of calculating the damagerate of the construction in time of manufacture. As shown in thediagram, since the internal stress of the diaphragm B per se is reducedwhen the SOI wafer is used, the bending amount of the diaphragm B can bereduced compared with the case of using the SOI structure wafer.Particularly, it is desirable from the viewpoint of securing mechanicalstrength that the thickness of the back electrode C is 5 μm or more.

(3) In the sound detecting mechanism according to the present invention,the material for the diaphragm B is not limited to polycrystal siliconor the active layer. The diaphragm B may be formed using a film havingconductivity such as a metal film, or a construction including a filmhaving conductivity laminated on an electrical insulating film such as aresin film. Specifically, it may be possible to use a high melting pointmaterial including tungsten when using a metal film.

(4) As described above, the present invention is aimed at reducing(restraining) the stress acting on the diaphragm B by selecting thethickness of the back electrode C. In addition to the constructionhaving the back electrode C with an increased thickness in this way, thestress acting on the diaphragm B may also be controlled by applyingimpurity diffusion to the diaphragm B. In one specific example of suchtreatment, boron is introduced into the polycrystal silicon film 302forming the diaphragm B by ion implantation technique with the energy of30 kV and a dose of 2E16 cm⁻². Heat treatment is executed at 1150° C. ina nitrogen atmosphere for eight hours as an activated heat treatment,thereby to form the diaphragm B having a compressed stress. Therefore,by combination of the film thickness ratio between the silicon oxidefilm and the silicon nitride film acting as the stop layer for siliconetching by the alkali etching liquid with impurity diffusion as well asthe thickness of the back electrode, the tension of the diaphragm B issynthetically controlled to balance the stress acting on the diaphragm Bwith the tension, which can release the tension acting on the diaphragmB and form the diaphragm B on which the necessary tension is exerted.

(5) It is also possible to form an integrated circuit on the supportsubstrate A constituting the sound detecting mechanism. The integratedcircuit functions to convert variations of capacitance between thediaphragm B and the back electrode C into electric signals for output.With the construction having such an integrated circuit, a take-outelectrode 305 may be connected with the integrated circuit through abonding wire or the like thereby to electrically connect between thediaphragm B, the back electrode C and the integrated circuit. With thisconstruction, there is no need to form an electric circuit on theprinted board or the like for converting variations of capacitancebetween the diaphragm B and the back electrode C into electric signalsfor output. This can minimize the size of the device and simplify theconstruction utilizing the sound detecting mechanism having thearrangement of the present invention.

According to the present invention, it is possible to provide a sounddetecting mechanism which forms a diaphragm with a required thickness bythickness control and yet restrains distortion of the diaphragm toprovide high sensitivity. This sound detecting mechanism may also beused as a sensor responsive to variations in aerial vibration and airpressure, besides a microphone.

1. A sound detecting mechanism comprising a pair of electrodes forming acapacitor on a substrate in which one of the electrodes is a backelectrode forming perforations therein corresponding to acoustic holesand the other of the electrodes is a diaphragm, wherein a multilayeredassembly is mounted on the substrate, the multilayered assembly formedof the diaphragm, a sacrificial layer and the back electrode superposedin series by vapor deposition technique; the sacrificial layer is etchedrelative to the multilayered assembly formed of the diaphragm, thesacrificial layer and the back electrode, thereby defining a void areabetween the diaphragm and the back electrode, with the sacrificial layerremaining at outer peripheral portions of the void area; and the backelectrode being formed by polycrystal silicon of 5 μm to 20 μm inthickness; and the substrate comprises a single crystal silicon oninsulator (SOI) structure wafer including a silicon oxide film or asilicon nitride film formed on a monocrystal silicon substrate and apolycrystal silicon film formed on the silicon oxide film or the siliconnitride film.
 2. The sound detecting mechanism of claim 1, wherein thesubstrate comprises a support substrate having a monocrystal siliconsubstrate acting as the base thereof, and a (100) silicon substrate isused as the monocrystal silicon substrate.
 3. The sound detectingmechanism of claim 1, wherein an impurity diffusion treatment isexecuted on the diaphragm.
 4. The sound detecting mechanism of claim 1,wherein the substrate comprises a support substrate having a monocrystalsilicon substrate acting as the base thereof; and the support substrateconsists of a single crystal silicon on insulator (SOI) wafer.
 5. Thesound detecting mechanism of claim 4, wherein the single crystal siliconon insulator (SOI) wafer has an active layer used as the diaphragm. 6.The sound detecting mechanism of claim 4, wherein the diaphragm isformed of monocrystal silicon of 0.5 μm to 5 μm in thickness.
 7. Thesound detecting mechanism of claim 1, wherein the polycrystal siliconfilm formed on the single crystal silicon on insulator (SOI) structurewafer is used as the diaphragm.
 8. The sound detecting mechanism ofclaim 1, wherein the diaphragm is formed of polycrystal silicon of 0.5μm to 5 μm in thickness.